70 THE CAUSES OF FLUCTUATIONS IN TUEGESCENCE
of the inferior one are. There is thus ample provision for a relative excess of
d i u r n a l turgescence iu it, and this, i a co-operation with the diurnal rise in leverage
i n t h e distal parts of t h e leaf accompanylDg solar stimulation, is q u i t e safficicut to account
f o r the occurrence of diurnal divergence. The inferior mass shows no distinct evidence
of a n y considerable excess i n relative straccural s t r e n g t h ; but it is aided in its t e n d e n cy
t o give rise t o convergence b y the strength of t h e axial mass of wood and medullary
tissue, and consequently nocturnal convergence is for a time extreme. The wood app
a r e n t l y plays a much more important part here in determining the assumption
of the nocturnal position than it does iu the case of the secondary and t e r t i a ry
p u l v i u i . The struggle in the p r i m a r y pulvini is, to a g r e a t extent, not so much between
t h e opposed masses of pulvinar parenchyma, as between the superior mass which tends to
cause divergence and the wood which makes for convergence in tho effort to regain its
o r i g i n a l dii'ection in r e l a t i o n to the axis. For a time the nocturnal loss in turgescence
i n the superior masi of pulvinar parenchyma, and the coincident d i m i n u t i o n in leverage
of t h e distal p o r t i o n s of t h e leaf, a l l ow t h e s t r o n g woody tissue t o resume i t s o r i g i n a l d n e c t i on
to the axis, and even to pass beyond it owing to the a d j u v a n t action of t h e i n f e r i or
p a r c u c h y m a ; but the movement r a p i d l j ' diminishes and ultimately disappears with the
c o n t i n u e d increase in bulk and s t r u c t u r al s t r e n g t h of the superior parenchyma, leaving
t h e p r i m a r y petiole in a permanent position of extreme divergence or even more or
less inclined backwards f r o m the line of the axis. So long as t h e movements continue
t o occur, they lead very conspicuously to that assumption of the maximal nocturnal
p o s i t i o n during the earlier part of the night, followed b y gradual depai-ture from it
i n spite of continued absence of solar stimulation, which, as has a l r e a d y been pointed
out, is 60 c h a r a c t e r i s t i c of n y c t i t r o p i c movements generally.
T h e pulvinar tissues in Pithecolobaan ai-e r e l a t i v e l y dense (the system of intercellular
spaces b e i n g coniparatively limited^, b u t p i t t i n g of t h e cell-walls is v e r y much more conspicuous
and abundant than it is in t h e pulviui of either of the species of Cassia (Plate V I I.
F i g s . 3, 8), and "with this we find a capacity for much more rapid movements in the
leaves. At a period in tho evening at which leaves of the species of Cassia are only
b e g i n n i n g to show indications of d e p a r t u r e f r om the diurnal position, those of Pithecolohium
saman ai'e a l r e a d y in t h e f u l l y developed nocturnal one. This may be p a r t i a l ly
due t o the g r e a t e r facilities for active t r a n s p i r a t i o n wMch t h e y present i n connection w th
thcii- gi-cat excess i n stoniata; but the fact t h a t similar phenomena present themselves in
connection w i t h continued a g i t a t i o n by showers of violent r a i n indicates that it is mainly
detei-mined b y textm-al facilities for r e d i s t r i b u t i o n of liquid. An amount of agitation by
w i n d or tropical showers wWch hai'dly produces any appreciable effects on Cassia ahta,
suffices t o produce maximal movement in the secondary petioles and pinnules of PithecoloUim.
The total amount of displacement involved in the t r a n s i t i o n f r om the diurnal
t o the nocturnal position iu both cases alike is v e r y g r e a t ; but tho r a t e at which it can
bo effected is v e i y different owing to the presence of structural peculiarities, which in
t h e case of Piihecolohium are of a nature to permit of redistribution of fluid by
filtrative discharge f r om the cell-cavities of the pulvinar pai-encliyma t a k i n g place much
snore r a p i d l y t h a n is possible iu the case of cither of the species of Cassia.
I n BO far as s t r i c t l y n y c t i t r o p i c movements—movements dependent on fluctuations
i n turgescence, detcimii led by vaiiations in degree of solar stimulation,—arc concerned,
another factor, however, comes into play besides tho mere structural facilities for the
escape of fluid from the cell cavities of the motor organs, and for transpii-atory loss
IN THE MOTOE OEOANS OF LEAVES. '
f r om fclie tissues generally. There can be no question that the puhdnar tissues iii
Mimosa púdica provide much greater facihties for filtrative redistribution of fluid tlian
t h o s e of Piihceolobiim saman do, and t h a t the lamin® contain an excess of s t o m a t a ; and
w i t h this we find a corresponding increase in c a p a c i t y for rapid movement in connection
w i t h tho incidence of mechanical agencies favouring increased local filtration owing
t o increased pressure. Eut yet the assumption of the nocturnal position occurs later
under normal conditions in Mimosa than in Piihecololium. The only explanation^ for
t h i s apparent anomaly must lie either i n differences i n t h e s t a b i l i t y of the osmotic prod
u c t s which are foi-med under the influence of solai- stimulation of the tissues, or iu
d i f f e r e n c e s in tho degree of protoplasmic stimulation and a c t i v i t y attending particular
degrees of i l l u m i n a t i o n in the two cases ; a u d as we find that t h e leaves of 31imosa not
o n r y assume the nocturnal position later, but resume the diurual one earlier, than
those of Pithecolohium do, aud tliat when once t h e y have begun to show movements
i n either direction tho latter are executed with relatively great rapidity, tliere can be
l i t t l e doubt that the second explanation is the correct one. The activity of the pro-
Mimosa must apparently be fully called into play and maintained by
; of illumination which arc incapable of producing an equally stimulant action
o n those of Pithecolobium.
I n Leuccena glauca the movements in the secondary rachlses and pinnules present
a general resemblance to those o c c u n i n g in the con-esponding parís of the leaves of
Piihccolobium saman; but f r om the outset the p r i m a r y petioles, a l t h o u g h provided w i t h a
most conspicuous pulvinus, fail to exhibit a n y appreciable periodic movements, and
i n place of these show steadily progressive divergence and rotation. The absence of
periodic movements is no doubt partially, at all events, connected with the fact that
t h e pi-imary petiole is considerably weaker in relation to the other parts of the leaf
t h a n it is in the case of Pithecolobium. In the latter the p r i m a i y petiole contributes
about 20 per cent, to the total weight of the leaf, whilst in Leuccena it contributes
o n l y half as much; and, as tl.is difference coincides with corresponding differences in
t h e s t r e n g t h of the vascular axis of the p r i m a r y pulvinus, it is evident that in Leucccna
t h e action of the latter as a factor making for convergence in consequence of its
t e n d e n c y to resume its original convergent position in relation to the axis will be
p r o p o r t i o n a t e l y feeble. The leaves are ordinarily provided with f r o m four to seven
s e c o u d a i y raehises, each of which beara f r om nine to fifteen pairs of narrow pinnules
w h i c h are set on at compai-atively wide intei-vals; so that, when the leaf is in a cond
i t i o n of full expansion, each of them is completely isolated f r o m its neighbours, and
t h e r e is an absence of a n y overlapping of successive pairs l i k e that which is o r d i n a r i ly
p r e s e n t in the leaves of Mimosa púdica under similar circumstances.
Owing to the v e i y l a s habit of the plant, the gi'owing portions of the shoots
a r e almost i n v a r i a b l y more or less horizontally disposed, and it is in connection with
t h i s that a necessity for excessive rotation of the primaiy petioles has arisen.
R o t a t i o n does present itself in the case of certain leaves of Piihecolohium savian, but
t h e phenomenon is not of constant occurrence as it is in Lcuccena glauca, and its
predominance in the latter is owing to the horizontal disposition of the axis on
w h i c h the growing leaves are situated. The mature leaflets are p r a c t i c a l l y a i i a n g e d iu
t w o rows, one l y i n g on either side of the axis, and with t h e i r surfaces f a c i n g directly
upwards in t h e same d i r e c t i o n as t h o upper surface of the axis. This position, however,
n e c e s s a r i l y implies the antecedent occurrence of a varying, and in most cases excessive